The success of vaginal birth by use of trans-labial ultrasound plus vaginal examination and vaginal examination only in pregnant women with labor induction: a comparative study

Background Predicting the success of vaginal delivery is an important issue in preventing adverse maternal and neonatal outcomes. Thus, this study aimed to compare the success rate of vaginal birth by using trans-labial ultrasound and vaginal examination, and vaginal examination only in pregnant women with labor induction. Methods This was a comparative study including 392 eligible pregnant women with labor induction attending to a teaching hospital affiliated with Iran University of Medical Sciences from April to October 2018 in Tehran, Iran. Women were randomly assigned to two groups; the trans-labial ultrasound plus vaginal examination (group A), and the vaginal examination only (group B). Women were included in the study if they satisfied the following criteria: singleton pregnancy, 37 to 42 weeks of gestational age, fetal head presentation, a living fetus with no abnormalities, uncomplicated pregnancy, and no previous cesarean section or any uterine surgery. We used a partograph for both groups to assess the fetal head position and the fetal head station. In group 1, the Angle of Progression (AoP) and Rotation Angle (RA) were also assessed. Finally, the success and progression of vaginal delivery in two groups were compared by predicting the duration of delivery and mode of delivery. Results The findings showed that 8.68% of women in the trans-labial plus vaginal examination group delivered by cesarean section, while 6.13% in the vaginal examination only group delivered by cesarean section (P = 0.55). In women with cesarean section in positive fetal head stations, Angle of Progression (AoP) was significantly decreased ranging from 90 to 135 degrees compared to women who delivered vaginally (135–180 degrees; P <  0.001). In addition, the Rotation Angle (RA) was significantly decreased in women with cesarean section ranging from 0 to 30 degrees compared to women who delivered vaginally (60-90degrees; P <  0.001). Further analysis indicated that a higher risk of cesarean section was associated with vaginal examination only as compared to trans-labial ultrasound plus vaginal examination (HR: 8.65, P <  0.001). Conclusion Angle of Progression (AoP) and Rotation Angle (RA) indexes might be useful parameters to predict labor progression and successful vaginal delivery among women undergoing labor induction.

Cesarean section such as any other surgeries could lead to a variety of complications and was commonly performed for mothers with history of previous cesarean section, fetal distress, and prolonged labor [14]. To prevent prolonged labor and ending pregnancy with vaginal birth, some procedures such as induction with oxytocin and prostaglandins via monitoring the labor process were used [15].
Labor monitoring and predicting the likelihood of vaginal birth following induction has become important in this era of personalized medicine, not just from women's perspectives, but also to ensure optimal allocation of healthcare resources [16]. Particularly, labor is usually monitored to ensure that there are no signs of abnormal progress that might be harmful to mother or baby. The method most commonly used is routine vaginal examination (undertaken at regular time intervals) in order to provide information on cervical dilatation and the position of the baby [17]. However, evidence suggests that the manual vaginal examination might lead to errors (up to 88% of cases) and has limitations in indicating labor progression [18][19][20][21][22][23]. As such, management of childbirth needs new approaches and guidelines, exploiting objective indications for standardized quantitative monitoring, and appropriate medical decision making for early and correct identification of the most effective mode of delivery [18,24,25].
Recent studies demonstrated that ultrasound techniques are very helpful in the measurement of labor progression parameters [20,[25][26][27][28][29][30]. For instance, the evaluation of the Progression Angle (PA) or Angle of Progression (AoP), and Rotation Angle (RA) by trans-labial ultrasound imaging provides an objective, accurate, and reproducible method for determining fetal head progression during labor [27,31]. Since in Iran using trans-labial ultrasound is not common in hospitals for monitoring labor procedures, this study aimed to evaluate the success rate of vaginal birth using trans-labial ultrasound plus vaginal examination compared to vaginal examination only in pregnant women with labor induction.

Design and participants
This was a comparative study. The study was conducted from April to October 2018. The study participants consisted of a cohort of pregnant women candidates for labor induction attending to the maternity ward in a teaching hospital affiliated with the Iran University of Medical Sciences in Tehran, Iran. Candidates were selected based on the inclusion criteria. The inclusion criteria were: to be Iranian, singleton pregnancy, 37 to 42 weeks gestational age (with ultrasound history in the first or second trimester of pregnancy), fetal head presentation, a living fetus with no abnormalities (known for embryos along with ultrasound reports), uncomplicated pregnancy (including preterm labor, placental abruption, severe preeclampsia, marginal placenta previa), no history of previous cesarean section or any uterine surgery, estimated fetal weight less than 5 kg, and not being estimated maximum one station of the fetal head during labor (with regard to the unpredictable speed prenatal delivery in women specially in the precipitous labor). The exclusion criteria were: cephalo-pelvic disproportion, and unwillingness to participate in the study. We used two methods to collect the data: 'trans-labial ultrasound plus vaginal examination' , and 'vaginal examination only' . Thus, women were randomly assigned into two groups: trans-labial ultrasound plus vaginal examination (group A) and vaginal examination only (group B). As such a standard-sized paper marked A (trans-labial plus vaginal examination group) or B (vaginal examination only group), and folded to fit an envelope. Then, at the time of admission to the delivery room, one of us (ENKH), offered a blind envelop to women for group assignment.

Sample size
In order to detect 20% differences in success rate between group A and group B (P 1 = 90% success rate for group A and 70% success rate for group B) we used the following formula to estimate sample size. n = (Z α/2 + Z β ) 2 * [p 1 (1-p 1 ) + p 2 (1-p 2 )]/(p 1 -p 2 ) 2 .
Considering a power of 80% and type I error equal to 5%, a sample size of 196 women per each group was estimated.

Trans-labial ultrasound
a. Procedure: Pregnant women were placed in a lithotomic position while their bladder was empty. The main investigator (ENKH) placed a trans-labial ultrasound probe in the trans-labial space of women. All underwent trans-labial ultrasound were performed by the main investigator. The ultrasound scan was equipped with a real-time tracking algorithm designed to guide the operator through the acquisition, with an automatic identification of anatomic landmarks ( Fig. 1), in order to be able to measure fetal head station, fetal head position, Angle of Progression and, Rotation Angle during labor ( Fig. 2 and Fig. 3). The probe was the Canox probe 3.5 MHz. It allowed quantitative monitoring of labor instantly through an automatic description of ultrasound images (Fig. 4). Afterwards, the probe was enclosed in a latex glove covered with ultrasound gel and then placed between the labia, below the pubic symphysis. In each occasion ultrasound imaging (the first probe) was placed vertically and data of the Angle of Progression and fetal head station were recorded (Fig. 5). Consequently, the probe was rotated 90 degrees to the right hip of the women, and data from the fetal head position and Rotation Angle were recorded (Fig. 6). Repeated imaging was performed by recording the time of imaging and a partograph was recorded in order to identify the labor process including information on the fetal head station and position, Angle of Progression, and Rotation Angle ( Fig. 5 and Fig. 6). The trans-labial ultrasound was performed every 4 hours in the first stage of labor, and every 1-hour or more   Fig. 2) and the tangent line to the fetal head (yellow dotted line in Fig. 2). In fact, the AOP is a measure between a line placed through the midline of the pubic symphysis and a line running from the inferior apex of the symphysis pubis tangentially to the fetal skull ( Fig. 2 and Fig. 7). The machine also automatically calculates the fetal head station value of the images obtained.
To calculate the RA, the reference parameters (anterior-posterior axis of the mother's pelvis (blue dotted line in Fig. 3), the surrounding area of the fetal head (green circle in Fig. 3), and the midline of the fetal head (red dotted line in Fig. 3) were used. Then, three-dimensional reconstructions with a graphical and temporal representation of the measured parameters were provided ( Fig. 5 and Fig. 6). c. Normal progress in trans-labial ultrasound: The partograph has been heralded as one of the most important advances in modern obstetric care. The World Health Organization (WHO) advocates its use as a necessary tool in the management of labor. In the normal active phase of labor, plotting of the fetal head station will have a four-hour progress. If it lasts more than four-hour, labor may be prolonged [32]. d. Abnormal progress in trans-labial ultrasound: Abnormal labor included protracted and arrest disorders. Protraction disorders refer to protracted active phase descent. Descent 1 cm/h in primiparas and 2 cm/h in multiparas is protracted descent. Arrest disorders refer to the secondary arrest of the fetal head descent for more than 1 hour [33].

Vaginal examination
a. Procedure: Pregnant women were placed in a lithotomic position whilst their bladder was empty. In women who were able to pass urine, especially in the latent phase of labor, it was encouraged to do it before every manual vaginal examination or translabial ultrasound, and in those who had received anesthesia methods or were in the active phase of labor and were unable to do it physiological, urinary catheterization was used. Vaginal examination was performed by a midwife. The midwife was blinded to the study, and confirmed by three senior gynecologists (there was no extra touch. The gynecologists  just were observers in order to see if the midwife performs vaginal examination correctly). The findings of the examination, including information about fetal head station, fetal head position and labor process were recorded in the delivery form and partograph.
Vaginal examination was performed every 4 hours in the first stage of labor, and every 2-hour or more in the active phase of labor [15]. Vaginal examination data was carried out after each uterine contraction. b. Measurement: Partograph was drawn with the most important indicators of progress: the fetal head position and the fetal head station. Since we did not need other information such as cervical dilatation for this study, thus we did not record extra information. The rate of progression of the labor is presented in Fig. 5.

Outcome measure
The primary outcome of this study was the success in predicting vaginal delivery by either group. As such predicting delivery time, and mode of delivery were considered.

Statistical analysis
The findings were analyzed using descriptive statistics (mean, standard deviation, and frequency distribution).
To compare data between the two methods (trans-labial ultrasound plus vaginal examination and vaginal examination only) independent t-test, analysis of variance, and Chi-square as well as Mann-Whitney U test, and Kruskal-Wallis test were performed as appropriate. Sensitivity analysis was used in order to compare the two methods in predicting vaginal delivery time. Additionally, the Cox proportional hazards regression model was used to estimate the probability of delivery outcomes while controlling for independent variables. Independent variables included a number of demographic information (date of birth, mother's education level, mother's occupation, gestational age, weight gained during pregnancy, height, current weight, pregnancy stage, prescription of pain medication or non-receipt of pain medication and Bishop's score.) and risk factors (prolong labor, fetal distress due to abnormal fetal heart rate or umbilical cord prolapse and placental separation). Finally, survival analysis was performed to predict delivery time. Then we compared the prediction time between the two study groups by t-test. All statistical analysis was performed using the SPSS 16 software.

Participants
A total of 392 pregnant women were included in the study and they were randomly assigned into two groups. The mean age of participants in the 'trans-labial plus vaginal examination' group was 27.59 (SD = 6.24) years, and it was 27.49 (SD =6.44) years for the 'vaginal examination only' group. There were no significant differences between the two groups in terms of age. Also, there were no significant differences in mothers' education (P = 0.4),  gestational age (P = 0.8), and the number of pregnancies (P = 0.2) between the two groups ( Table 1).

Risk of cesarean section
The results obtained from Cox's proportional hazard for the probability of the outcome measure (type of delivery) among the two study groups are shown in Table 2.
As indicated there was a higher risk of cesarean section associated with vaginal examination only as compared to ultrasound labial plus vaginal examination (HR: 8.65, P < 0.001).

Prediction of duration of delivery mode
Survival analysis was used to predict the duration of delivery (in minutes) for the two groups. The duration was estimated from the beginning of induction to the time of delivery (whether vaginal or cesarean section  Table 3). The time-course analysis showed that the prediction of duration for vaginal delivery in the ultrasound method was lower than in the manual vaginal examination method at all instances.

Sensitivity analysis
The findings showed that the sensitivity for prediction of vaginal birth with manual vaginal examination was 75.54% and by trans-labial ultrasound it was 91.6%. The results also showed that the data obtained from translabial ultrasound were more sensitive in predicting vaginal birth, especially in zero and positive stations (Tables 4  and Table 5).

Progression angle and rotation angle
Finally, when the progression angle was measured in the trans-labial ultrasound group the findings indicated that for women whose progression angle in 0 and + 1 station   was 135 degrees, vaginal birth occurred (Fig. 8), while for women whose progression angle in 0 and + 1 stations was less than 135 degrees, cesarean section occurred (Fig. 9). Also, in stations + 2 and + 3, vaginal birth occurred for those women who had progression angle of 135 to180 degrees (Table 6). In addition, while the majority of women in the vaginal birth group (57.5%) had the final rotation angle of the fetal head within 60 to 90 degrees, the majority of women in the cesarean group (47.1%) had the final angle of 0 to 30 degrees (Table 7).

Discussion
The findings from this study indicated that trans-labial ultrasound was more effective than manual vaginal examination in the labor process, and accurate prediction of delivery types. Similar findings were reported from studies that assessed AoP and RA. They showed that transperineal ultrasound could successfully predict vacuum   (Table 6). However, by separating the fetal head station, we introduced some intervals for the Angle of Progression that would have increase the probability of vaginal birth with 91.6% sensitivity (Table 4). A recent study showed that if fetal head angle locates between 135 and 158 degrees in the second phase of labor, the probability of success significantly would increase for instrumental birth. The same study reported that when Angle of Progression was less than 135 degrees, the  Table 6 Comparison between head angle to the pubic bone center (angle progression) and sensitivity to the segregation of the fetal head and the prediction of the type of final delivery (cesarean/vaginal) in the two groups success of vaginal birth (with or without instrument) decreased [36].
The current study showed that sensitivity for translabial ultrasound was 91.6%, while sensitivity for the routine vaginal examination was 75.54% (Table 4, and  Table 5). Such findings suggest that trans-labial ultrasound was more accurate in predicting vaginal birth. Therefore, to assess labor progress it is recommended to use trans-labial ultrasound as an adjunct to routine vaginal or digital clinical examinations to reduce invasive procedures and human errors. Similarly, studies have shown that ultrasound, as a less invasive and reproducible method, was effective in predicting the success of vaginal birth in pregnant women compared to other methods [22,40,41]. As such studies reported that the accuracy of the ultrasound method did not significantly change, even if the examiners had different levels of proficiency [18,28,42,43] while this is not true for vaginal examination [44].
Although the present study was focused on assessing the prediction of duration of vaginal delivery and the AoP measurement by expert manual segmentation, we also found a good agreement between a range of AoP values and the occurrence of a spontaneous delivery. In particular, in the present study, for all women who delivered vaginally the value for AoP was greater than 135 degrees in the zero station, and 135 to 180 degrees in positive stations of the fetal head (Table 6). A recent study stated that the late diagnosis of posterior occiput position in the second stage of labor and obstructed labor is exposed to a high risk of maternal and fetal morbidity [45]. Moreover, other studies reported that using ultrasound technique and the earlier detection of the abnormal fetal head station is more precise than vaginal examination which might lead to maternal complications and maternal and neonatal unfavorable outcomes [21,37,46].
The findings from the current study showed that the majority of women with a final RA of the fetal head above 60 degrees had vaginal birth while the value for those who had cesarean section was less than 30 degrees (Table 7). However, a study reported that women with a final RA of the fetal head above 22 degrees experienced vaginal birth [41]. Another study stated that women with a final RA of the fetal head above 45 degrees experienced cesarean deliveries, and those with less than 45 degrees experienced vaginal birth [26]. Such differences might be due to differences in observation points in trans-labial ultrasound. Technicians might see the central point of the pubic bone while experienced sonographers usually measure the peripheral of the pubic bone. The other possibility for such observation was the fact that in the current study the data obtained from the processing of echo graphic images in the trans-labial ultrasound, automatically created the progress algorithms without human manipulation which significantly decreased human error.

Strengths and limitations
In order to measure AoP we used the center of pubis symphysis bone while previous studies used peripheral sides for such measure [24,36]. Using peripheral sides firstly needs a highly experienced expert and secondly might introduce more errors. In addition, the current study benefited from a relatively large sample size compared to previous investigations on the topic. However, the study had some limitations. Since this study was an observational study, there was a difference between women in the two study groups with regard to the number of pregnancies that might influence the findings. Studies with more homogenous samples are recommended.

Conclusion
Overall, the findings suggest that trans-labial ultrasound plays a significant role in the management of labor and helps to make the right decisions in the labor process. As such the Angle of progression (AoP) and Rotation Angle (RA) measured by trans-labial ultrasound were influential factors to predict vaginal birth or cesarean section in labor process. Indeed, further investigations on the use of trans-labial ultrasound for the prediction of successful vaginal birth are recommended.  Chi-square test results χ 2 =22.71, df = 7, P < 0.001